LED driver and LED driving method

ABSTRACT

A method of driving an LED can include: (i) controlling a main power switch by a control circuit of an LED driver; (ii) turning on an auxiliary power switch when the main power switch is turned on such that a DC bus voltage is provided to the main power switch through the auxiliary power switch, and the main power switch outputs a driving current to a load; (iii) charging a capacitor by the DC bus voltage through the auxiliary power switch when the main power switch is turned off, where the capacitor is charged until a voltage across the capacitor reaches a predetermined stable voltage; (iv) clamping the voltage across the capacitor at the predetermined stable voltage; and (v) using the clamping voltage across the capacitor as a supply voltage for the control circuit.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No.201410364402.X, filed on Jul. 28, 2014, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present disclosure relates to the field of power electronics, andmore particularly to an LED driver and associated driving method.

BACKGROUND

A switched-mode power supply (SMPS), or a “switching” power supply, caninclude a power stage circuit and a control circuit. When there is aninput voltage, the control circuit can consider internal parameters andexternal load changes, and may regulate the on/off times of the switchsystem in the power stage circuit. Switching power supplies have a widevariety of applications in modern electronics. For example, switchingpower supplies can be used to drive light-emitting diode (LED) loads.

SUMMARY

In one embodiment, a method of driving an LED can include: (i)controlling a main power switch by a control circuit of an LED driver;(ii) turning on an auxiliary power switch when the main power switch isturned on such that a DC bus voltage is provided to the main powerswitch through the auxiliary power switch, and the main power switchoutputs a driving current to a load; (iii) charging a capacitor by theDC bus voltage through the auxiliary power switch when the main powerswitch is turned off, where the capacitor is charged until a voltageacross the capacitor reaches a predetermined stable voltage; (iv)clamping the voltage across the capacitor at the predetermined stablevoltage; and (v) using the clamping voltage across the capacitor as asupply voltage for the control circuit, where the capacitor is coupledbetween an input terminal of the supply voltage and a control ground.

In one embodiment, an LED driver can include: (i) a main power switchand a control circuit, where an output terminal of the control circuitis coupled to a gate of the main power switch for controlling switchingstates of the main power switch, and where the main power switch iscoupled to a load; (ii) an auxiliary power switch coupled to a DC busvoltage and the main power switch; (iii) a capacitor coupled between asupply voltage of the control circuit and a control ground, where avoltage across the capacitor is configured as the supply voltage; (iv) avoltage-stabilizing circuit coupled to the supply voltage, and beingconfigured to clamp the supply voltage to a predetermined stable voltagewhen the supply voltage reaches a level of the predetermined stablevoltage; and (v) a supply voltage control circuit coupled between theauxiliary power switch and the supply voltage, where the DC bus voltageis configured to charge the capacitor through the auxiliary power switchand the supply voltage control circuit when the main power switch isoff, and where the DC bus voltage is provided to the main power switchthrough the auxiliary power switch, and a driving current output fromthe main power switch is configured to drive the load when the mainpower switch and the auxiliary power switch are on.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a first example LED driver, inaccordance with embodiments of the present invention.

FIG. 2 is a flow diagram of an example method of driving an LED, inaccordance with embodiments of the present invention.

FIG. 3 is a schematic block diagram of a second example LED driver, inaccordance with embodiments of the present invention.

FIG. 4 is a schematic block diagram of a third example LED driver, inaccordance with embodiments of the present invention.

FIG. 5 is a schematic block diagram of a fourth example LED driver, inaccordance with embodiments of the present invention.

FIG. 6 is a schematic block diagram of a fifth LED driver, in accordancewith embodiments of the present invention.

FIG. 7 is a schematic block diagram of a control circuit for an LEDdriver, in accordance with embodiments of the present invention.

FIG. 8 is a structure diagram of an example integrated circuit chip foran LED driver, in accordance with embodiments of the present invention.

FIG. 9 is a circuit structure diagram employing the integrated circuitchip of FIG. 8, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

Reference may now be made in detail to particular embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention may be described in conjunction with thepreferred embodiments, it may be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents that may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it may be readilyapparent to one skilled in the art that the present invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, processes, components, structures, and circuitshave not been described in detail so as not to unnecessarily obscureaspects of the present invention.

Light-emitting diode (LED) lights can employ improved drivingtechniques, and may be widely used in various fields. An LED driver canbe used to drive an LED load with a substantially constant current. AnLED driver can include a power stage circuit and a control circuit. Thecontrol circuit may be used to intermittently turn on a main powerswitch in the power stage circuit, so as to convert an input voltage ofthe power stage circuit into a constant current for an LED load (e.g., alight). Various demands on LED drivers include smaller volume, lowerproduction costs, and longer life time.

Referring now to FIG. 1, shown is a schematic block diagram of a firstexample LED driver, in accordance with embodiments of the presentinvention. In this particular example, the main power stage circuit maybe simplified to a high voltage depletion mode power transistor, suchthat the control circuit and the power transistor can be integrated intoa high power density chip (e.g., a control chip). Therefore, the circuitvolume may be largely reduced via an integrated circuit in thisapproach. However, an additional power supply circuit may be needed forthis particular example in order to supply an operating voltage to thecontrol circuit. In addition, a turning-on control signal for the mainpower switch may be obtained by sampling DC bus voltage Vin throughvoltage division resistors Rvs1 and Rvs2. While this approach isrelatively simple, product costs and circuit volume may be increased dueto the use of high voltage resistors.

In one embodiment, a method of driving an LED can include: (i)controlling a main power switch by a control circuit of an LED driver;(ii) turning on an auxiliary power switch when the main power switch isturned on such that a DC bus voltage is provided to the main powerswitch through the auxiliary power switch, and the main power switchoutputs a driving current to a load; (iii) charging a capacitor by theDC bus voltage through the auxiliary power switch when the main powerswitch is turned off, where the capacitor is charged until a voltageacross the capacitor reaches a predetermined stable voltage; (iv)clamping the voltage across the capacitor at the predetermined stablevoltage; and (v) using the clamping voltage across the capacitor as asupply voltage for the control circuit, where the capacitor is coupledbetween an input terminal of the supply voltage and a control ground.

Referring now to FIG. 2, shown is a flow diagram of an example method ofdriving an LED, in accordance with embodiments of the present invention.At 201, a DC bus voltage can be received. Also, a main power switch canbe controlled via a control circuit. For example, the DC bus voltage canbe obtained by rectifying an AC input power supply through a rectifierbridge. At 202, and auxiliary power switch can be turned on when themain power switch is turned on. In this case, the DC bus voltage canoutput a driving current to a load (e.g., an LED load) through theauxiliary power switch and the main power switch. At 203, a capacitorcan be charged by the DC bus voltage until a voltage across thecapacitor reaches a predetermined stable voltage when the main powerswitch is turned off. Also, the voltage across the capacitor can beclamped to a stable voltage (e.g., a predetermined voltage level).

The control circuit may generate a control signal for the main powerswitch, in order to control on/off states of the main power switchduring normal operation of an LED driver. When the control signal isactive, the main power switch can be turned on. When the auxiliary powerswitch is turned on, the power stage circuit of the LED driver can beformed by the auxiliary power switch and the main power switch coupledin series between an input terminal of the DC bus voltage and the load.In this case, the DC bus voltage can output the driving current to theload through the power stage circuit formed by the auxiliary powerswitch and the main power switch. Of course, the driving current may beutilised to drive LEDs when LEDs are employed as the load.

When the control signal is inactive, the main power switch can be turnedoff. In a predetermined time interval after the main power switch isturned off, the auxiliary power switch can remain on. In this case, theDC bus voltage may charge a capacitor that is coupled between a supplyvoltage input terminal and a control ground of the control circuit ofthe LED driver through the auxiliary power switch. During the chargingprocess, the voltage across the capacitor can increase (e.g., the supplyvoltage rises). Because of the voltage-stabilizing circuit that iscoupled to the capacitor, the supply voltage can be clamped at a stablevoltage such that the voltage across the capacitor can be used as thesupply voltage of the control circuit during operation of the LEDdriver. Therefore, in particular embodiments, the LED driver can beself-powered, without any other power supply circuitry.

In one example, a starting resistor can be configured between the inputterminal of the DC bus voltage and the capacitor. The DC bus voltage cancharge the capacitor through the starting resistor in order to providethe supply voltage during a start-up (e.g., initialization or power up)phase of the LED driver. For example, during charging of the capacitor,the charging current for the capacitor can be limited to ensure that thecharging current is less than an upper limit of a predetermined chargingcurrent. In this way, the capacitor may be protected by avoiding apotentially too high or damaging charging current.

In another example, during the charging process of the capacitor, whenthe charging current is greater than the upper limit of thepredetermined charging current, over-current protection can beaccommodated. In this case, the charging current may be disconnected byturning off the auxiliary power switch in order to effectively cut offthe connection between the auxiliary power switch and the capacitor. Inthis way, the charging process of the capacitor can be disabled toprotect the capacitor by avoiding charging current that is too high.

Also for example, operation of the LED driver, the DC bus voltage inputto the LED driver can be monitored. If the DC bus voltage is greaterthan an upper limit of the input voltage (e.g., if an over voltagecondition occurs), the auxiliary power switch and/or the main powerswitch can be turned off to protect the LED driver and the load. Whetherthe DC bus voltage is in an over voltage condition can be determinedbased on the gate voltage of the auxiliary power switch. The gatevoltage can represent energy transferred to the auxiliary power switchby the parasitic capacitor thereof. When the gate voltage is greaterthan a predetermined voltage, an over voltage can be determined, andover voltage protection can begin by controlling a turn off theauxiliary power switch and/or the main power switch. In another example,the over voltage condition can be determined by monitoring a “jumpslope” or transition time of a drain-source voltage of the main powerswitch. When such a jump slope is greater than a predetermined jumpslope, over voltage protection can begin.

The control circuit of this particular example can control the turn onand off the main power switch according to the voltage input to the mainpower switch, and the saturated current flowing through the main powerswitch. For example, when the voltage input to the main power switch isless than a predetermined voltage, the control signal can be activatedto turn on the main power switch. When the current flowing through themain power switch is saturated, the control signal can be deactivated toturn off the main power switch. In this way, the control circuit cancontrol the driving current for the LED load by turning on/off the mainpower switch.

In particular embodiments, when the main power switch and the auxiliarypower switch are turned on, the series-connected main power switch andauxiliary power switch may form the power stage circuit. In this case,the DC bus voltage can output the driving current for the LED loadthrough the power stage circuit. Further, transistors can be used toimplement the auxiliary power switch and the main power switch.Moreover, when the main power switch is turned off, the DC bus voltagecan charge a capacitor through the auxiliary power switch, and thevoltage across the capacitor may be used as the supply voltage of thecontrol circuit. When the voltage across the capacitor reaches apredetermined stable voltage, the voltage across the capacitor can beclamped at the stable voltage. Thus, the LED driver can be self-poweredwithout any other power supply circuitry in order to simplify the LEDdriver circuit structure.

In one embodiment, an LED driver can include: (i) a main power switchand a control circuit, where an output terminal of the control circuitis coupled to a gate of the main power switch for controlling switchingstates of the main power switch, and where the main power switch iscoupled to a load; (ii) an auxiliary power switch coupled to a DC busvoltage and the main power switch; (iii) a capacitor coupled between asupply voltage of the control circuit and a control ground, where avoltage across the capacitor is configured as the supply voltage; (iv) avoltage-stabilizing circuit coupled to the supply voltage, and beingconfigured to clamp the supply voltage to a predetermined stable voltagewhen the supply voltage reaches a level of the predetermined stablevoltage; and (v) a supply voltage control circuit coupled between theauxiliary power switch and the supply voltage, where the DC bus voltageis configured to charge the capacitor through the auxiliary power switchand the supply voltage control circuit when the main power switch isoff, and where the DC bus voltage is provided to the main power switchthrough the auxiliary power switch, and a driving current output fromthe main power switch is configured to drive the load when the mainpower switch and the auxiliary power switch are on.

Referring now to FIG. 3, shown is a schematic block diagram of a secondexample LED driver, in accordance with embodiments of the presentinvention. This particular example LED driving circuit can include mainpower switch QM, auxiliary power switch Q1, capacitor C1,voltage-stabilizing circuit 303, supply voltage control circuit 304, anda control circuit (not shown in FIG. 3). A drain electrode of auxiliarypower switch Q1 can connect to input terminal IN of DC bus voltage Vin,and a source electrode of auxiliary power switch Q1 can connect to adrain electrode of main power switch QM. That is, auxiliary power switchQ1 can connect in series between the input terminal (IN) of DC busvoltage Vin, and main power switch QM. The source electrode of mainpower switch QM can be coupled to load 305, and may provide drivingcurrent Io to load 305.

Capacitor C1 can connect between supply voltage Vcc of the controlcircuit and control ground Vss, and the voltage across capacitor C1 canbe configured as supply voltage Vcc of the control circuit. Voltagestabilizing circuit 303 can be coupled to the input terminal of supplyvoltage Vcc, and supply voltage control circuit 304 may be coupledbetween auxiliary power switch Q1 and the input terminal of supplyvoltage Vcc. The output terminal of the control circuit can be coupledto the control terminal of main power switch QM. During normal LEDdriver operation, the control circuit can control the main power switchto be turned on/off with a predetermined control principle. When mainpower switch QM and auxiliary power switch Q1 are both on, DC busvoltage Vin can provide driving current Io through a power stage circuitthat includes auxiliary power switch Q1 and main power switch QM, inorder to drive load 305.

During a predetermined time interval after main power switch QM isturned off, auxiliary power switch Q1 can remain on, and supply voltagecontrol circuit 304 may be enabled. In this case, DC bus voltage Vin cancharge capacitor C1 through auxiliary power switch Q1 and supply voltagecontrol circuit 304. During the charging process, supply voltage Vcc mayincrease. When supply voltage Vcc rises to a level of the predeterminedstable voltage of voltage-stabilizing circuit 303, the voltage of supplyvoltage Vcc (e.g., the voltage across the capacitor C1) can be clampedto a level of the stable voltage.

In this fashion, when main power switch QM and auxiliary power switch Q1are turned on, the series-connected main power switch and auxiliarypower switch may form the power stage circuit. In this case, DC busvoltage Vin can output driving current Io for the LED load through thepower stage circuit. Also, transistors can be used to implement theauxiliary power switch Q1 and the main power switch QM, rather thanusing a high voltage depletion mode device. When main power switch QM isturned off, DC bus voltage Vin can charge capacitor C1 coupled betweenthe input terminal of supply voltage Vcc and the control ground throughauxiliary power switch Q1 and supply voltage control circuit 304. Thevoltage across capacitor C1 can be configured as supply voltage Vcc ofthe control circuit. When the voltage across capacitor C1 reaches alevel of the predetermined stable voltage, voltage-stabilizing circuit303 coupled to the input terminal of supply voltage Vcc can be utilizedto clamp the voltage of capacitor C1 at the stable voltage. In this way,the LED driver of particular embodiments can be self-powered withoutfurther external supply voltage circuitry.

In certain embodiments, configuration of the LED driver can besimplified in order to reduce circuit volume and product costs, and tofacilitate integrated circuit design. In the example of FIG. 3,rectifier bridge 302 can be utilized to rectify AC voltage Vacin, and toobtain DC bus voltage Vin. Also, zener diode Dz may be utilized asvoltage-stabilizing circuit 303, with an anode connected to a commonterminal of main power switch QM and load 305, and a cathode connectedto the input terminal of supply voltage Vcc.

In the example of FIG. 3, unidirectional conduction circuits 3041 and3042 may be used to form supply voltage control circuit 304.Unidirectional conduction circuits 3041 and can include diodes D1 and D2as shown. The anode of diode D1 can connect to the source of auxiliarypower switch Q1. The anode of diode D2 can connect to supply voltageVcc, and the cathode of diode D2 can connect to the gate of auxiliarypower switch Q1. When main power switch QM is turned on, drain voltageVdrain of main power switch QM can be clamped to the zero potential ofcontrol ground Vss. Thus, the source voltage of auxiliary power switchQ1 that is coupled to the drain of main power switch QM can also beclamped to the zero potential.

In this case, unidirectional conduction circuit 3041 (e.g., includingdiode D1) may be off, and unidirectional conduction circuit 3042 (e.g.,including diode D2) can be turned on due to the function of supplyvoltage Vcc. The gate voltage of auxiliary power switch Q1 may beclamped at supply voltage Vcc, and gate-source voltage Vgs of auxiliarypower switch Q1 can be supply voltage Vcc. Thus, auxiliary power switchQ1 may be turned on and may form a power stage circuit with main powerswitch QM. DC bus voltage Vin can provide stable driving current Io forload 305 through such a power stage circuit.

During the time interval when main power switch QM is turned off, drainvoltage Vdrain of main power switch may gradually rise. When drainvoltage Vdrain rises to be slightly greater than supply voltage Vcc,unidirectional conduction circuit 3041 can be turned on, and capacitorC1 can begin to charge. In this case, supply voltage Vcc may graduallyrise, and because of voltage-stabilizing circuit 303 (e.g., includingzener diode Dz), capacitor C1 may stop charging when supply voltage Vccrises to a level of the predetermined stable voltage (e.g., Uz).

In the example of FIG. 3, current limiter 3043 can also be included inunidirectional conduction circuit 3041. Current limiter 3043 can connectin series to diode D1, and during charging of capacitor C1, currentlimiter 3043 may limit the charging current to not go higher than anupper limit of the predetermined charging current, for circuitprotection. For example, when the charging current of capacitor C1 isgreater than the upper limit of the predetermined charging current,current limiter 3043 and unidirectional conduction circuit 3041 can beturned off such that charging of capacitor C1 is stopped/disabled.

Referring now to FIG. 4, shown is a schematic block diagram of a thirdexample LED driver, in accordance with embodiments of the presentinvention. In this particular example resistor R1 can connect in serieswith diode D1 in unidirectional conduction circuit 3041. When switch Q2(e.g., a transistor) is configured as unidirectional conduction circuit3042, the drain of switch Q2 can connect to the gate of auxiliary powerswitch Q1, and the source and gate of switch Q2 can connect to the twoterminals of resistor R1. Thus, the gate-source voltage of switch Q2 canequal the voltage across resistor R1. During the charging of capacitorC1, the charging current may flow through resistor R1, and the voltageacross resistor R1 is larger as the charging current increases. When thecharging current reaches the upper limit, the voltage across resistor R1may be increased to reach the driving voltage of switch Q2. Thus, switchQ2 may be turned on, and the voltage across resistor R1 can be providedbetween the gate and source of auxiliary power switch Q1 to turn offpower switch Q1. This can stop charging capacitor C1, in order to avoida charging current that is too high from flowing to capacitor C1, so asto achieve current limiting protection.

When main power switch QM is turned on, the source voltage of auxiliarypower switch Q1 may be pulled down to the zero potential of the controlground. In this case, unidirectional conduction circuit 3041 may be off,switch Q2 may be on, and the gate-source voltage of auxiliary powerswitch Q1 can equal to supply voltage Vcc. Thus, auxiliary power switchQ1 can be turned on, and DC bus voltage Vin may output driving currentIo through auxiliary power switch Q1 and main power switch QM.

When the system quickly switches between on and off states, becausecapacitor C1 is coupled to the input terminal of supply voltage Vcc, theenergy stored in capacitor C1 can maintain the LED driver as a normaloperation for a time interval when the input voltage is off-line. Duringthis time period or interval, if main power switch QM is on, relativelylarge current may flow through main power switch QM when the inputvoltage is a relatively high voltage, and this can potentially harm theLED driver and/or load 305. In order to address such a case, the LEDdriver may further include an over power protection monitor that can beused to turn off auxiliary power switch Q1 and/or main power switch QMwhen the current DC bus voltage Vin is greater than the upper limit ofthe predetermined input voltage (e.g., an over voltage condition). Inthis way, the input of DC bus voltage Vin can be cut off so as toprotect the LED driver and load 305.

Referring now to FIG. 5, shown is a schematic block diagram of a fourthexample LED driver, in accordance with embodiments of the presentinvention. In this particular example, over power protection monitor 502can connect to the gate of auxiliary power switch Q1. Over powerprotection monitor 502 may also have parasitic capacitor Cgd connectedto the gate of auxiliary power switch Q1, and the gate voltage maychange along with the energy input to auxiliary power switch Q1. Thus,the gate voltage of auxiliary power switch Q1 can be used to determineif DC bus voltage Vin of auxiliary power switch Q1 is over voltage. Ifyes, over power protection monitor 502 can turn off main power switch QMand/or auxiliary power switch Q1, in order to protect the LED driver andload 305. In addition, a resistor (not shown) can connect between thegate of auxiliary power switch Q1 and the drain of switch Q2. When DCbus voltage Vin suddenly increases, the voltage on the resistor may alsosuddenly increase, and the over power protection can protect the LEDdriver and load 305.

Referring now to FIG. 6, shown is a schematic block diagram of a fifthLED driver, in accordance with embodiments of the present invention. Inthis particular example, over power protection monitor 602 can connectbetween the drain and the source of main power switch QM. Over powerprotection monitor 602 can detect a slope change of drain-source voltageVds of main power switch QM. For example, it can be determined that DCbus voltage Vin is in an over voltage condition when the slope change isgreater than a predetermined value, and over power protection mayaccordingly to protect the LED driver and load 305. In this way, circuitreliability can be improved, the circuit structure can be simplified,and the circuit volume can be reduced for over power protection.

Referring now to FIG. 7, shown is a schematic block diagram of a controlcircuit for an LED driver, in accordance with embodiments of the presentinvention. In this particular example, control circuit 701 can includecontrol signal generator 702, feedback compensation circuit 703, anddriving circuit 704. Control signal generator 702 can connect to thedrain of main power switch QM to receive drain voltage Vdrain andsaturation current IMs of main power switch QM, and may generate controlsignal Vc for controlling main power switch QM. For example, controlsignal Vc can be activated when drain voltage Vdrain of main powerswitch QM is less than a predetermined voltage, for turning on mainpower switch QM. When the current of main power switch QM is saturated,control signal Vc can be inactive, so main power switch QM may be turnedoff. In this way, the switching states of main power switch QM may becontrolled, so as to control driving current Io for the LED load.

For example, drain voltage Vdrain of the main power switch that mayrepresent current DC bus voltage Vin can be sampled as a trigger signalfor controlling the switching states of main power switch QM. Thetrigger signal can be sampled inside the integrated circuit chip.Referring back to FIGS. 3-6 along with FIG. 7, output current samplingresistor R2 can connect between main power switch QM and load 305. Sensevoltage signal Vis on the output current sampling resistor R2 thatrepresents driving current Io can be sampled and provided to controlcircuit 701.

Also, current feedback compensation circuit 703 in control circuit 701may generate feedback compensation signal Vcomp according to sensevoltage signal Vis. Further, driving circuit 704 can generate controlsignal Vgate for controlling main power switch QM according to controlsignal Vc and feedback compensation signal Vcomp. In addition, oralternatively, starting resistor R3 can connect between the inputterminal of DC bus voltage Vin and capacitor C1. The DC bus voltage Vinmay charge capacitor C1 through starting resistor R3 in order to providethe supply voltage during a start-up period of the LED driver.

Referring now to FIG. 8, shown is a structure diagram of an exampleintegrated circuit chip for an LED driver, in accordance withembodiments of the present invention. For example, circuits 301, 401,501, and 601 shown in FIGS. 3-6 can be integrated in the chip shown inFIG. 8. In one particular example, the chip of FIG. 8 can be in the formof an SOT2-3; however, other configurations or types of chips can alsobe supported in certain embodiments.

Referring now to FIG. 9, shown is a circuit structure diagram employingthe integrated circuit chip of FIG. 8, in accordance with embodiments ofthe present invention. The example chip of FIG. 8 can be coupled toperipheral circuits in the example FIG. 9. In this way, an LED driver ofparticular embodiments may have a simplified circuit structure, as wellas reduced circuit volume and product costs, as compared to conventionalapproaches.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with modifications as are suited to particularuse(s) contemplated. It is intended that the scope of the invention bedefined by the claims appended hereto and their equivalents.

What is claimed is:
 1. A method of driving a light-emitting diode (LED)load, the method comprising: a) controlling a main power switch by acontrol circuit of an LED driver; b) turning on an auxiliary powerswitch when said main power switch is turned on such that a DC busvoltage is provided to said main power switch through said auxiliarypower switch, and said main power switch outputs a driving current tosaid LED load; c) charging a capacitor by said DC bus voltage throughsaid auxiliary power switch when said main power switch is turned off,wherein said capacitor is charged until a voltage across said capacitorreaches a predetermined stable voltage; d) clamping, by a zener diode,said voltage across said capacitor at said predetermined stable voltage,wherein an anode of said zener diode is directly connected to a commonnode of said main power switch and an output current sampling resistor,wherein said output current sampling resistor is directly connected tosaid LED load; and e) using said clamping voltage across said capacitoras a supply voltage for said control circuit, wherein said capacitor iscoupled between an input terminal of said supply voltage and a controlground.
 2. The method of claim 1, wherein said charging said capacitorfurther comprises limiting a charging current to guarantee that saidcharging current is less than an upper limit of a predetermined chargingcurrent.
 3. The method of claim 2, wherein said charging said capacitorfurther comprises cutting off said charging current when said chargingcurrent is greater than said upper limit of said predetermined chargingcurrent.
 4. The method of claim 3, wherein said cutting off saidcharging current comprises at least one of turning off said auxiliarypower switch, and disconnecting said auxiliary power switch from saidcapacitor.
 5. The method of claim 1, wherein controlling said main powerswitch by said control circuit comprises: a) turning on said main powerswitch when a voltage input to said main power switch through saidauxiliary power switch is less than a predetermined voltage; and b)turning off said main power switch when a current of said main powerswitch is saturated.
 6. The method of claim 5, wherein: a) a drain ofsaid main power switch is coupled to an input terminal of said DC busvoltage through said auxiliary power switch; and b) said main powerswitch is turned on when a drain voltage of said main power switch isless than said predetermined voltage.
 7. A light-emitting diode (LED)driver for an LED load, the LED driver comprising: a) a main powerswitch and a control circuit, wherein an output terminal of said controlcircuit is coupled to a gate of said main power switch for controllingswitching states of said main power switch, and wherein said main powerswitch is coupled to said LED load; b) an auxiliary power switch coupledto a DC bus voltage and said main power switch; c) a capacitor coupledbetween a supply voltage of said control circuit and a control ground,wherein a voltage across said capacitor is configured as said supplyvoltage; d) a voltage-stabilizing circuit comprising a zener diodecoupled to said supply voltage, and being configured to clamp saidsupply voltage to a predetermined stable voltage when said supplyvoltage reaches a level of said predetermined stable voltage, wherein ananode of said zener diode is directly connected to a common node of saidmain power switch and an output current sampling resistor, wherein saidoutput current sampling resistor is directly connected to said LED load;and e) a supply voltage control circuit coupled between said auxiliarypower switch and said supply voltage, wherein said DC bus voltage isconfigured to charge said capacitor through said auxiliary power switchand said supply voltage control circuit when said main power switch isoff, and wherein said DC bus voltage is provided to said main powerswitch through said auxiliary power switch, and a driving current outputfrom said main power switch is configured to drive said LED load whensaid main power switch and said auxiliary power switch are on.
 8. TheLED driver of claim 7, wherein said supply voltage control circuitcomprises: a) a first unidirectional conduction circuit having a firstterminal coupled to a first terminal of said main power switch, a secondterminal coupled to an input terminal of said supply voltage, whereinsaid first unidirectional conduction circuit is turned on when a voltageat said first terminal of said first unidirectional conduction circuitis higher than a voltage at said second terminal; and b) a secondunidirectional conduction circuit having a first terminal coupled tosaid input terminal of said supply voltage, a second terminal coupled toa gate of said auxiliary power switch, wherein said secondunidirectional conduction circuit is turned on when a voltage at saidfirst terminal of said second unidirectional conduction circuit ishigher than a voltage at a second terminal.
 9. The LED driver of claim8, wherein: a) said first unidirectional conduction circuit comprises afirst diode having an anode coupled to said first terminal of said mainpower switch, and a cathode coupled to said input terminal of saidsupply voltage; and b) said second unidirectional conduction circuitcomprises a second diode having an anode coupled to said input terminalof said supply voltage, and a cathode coupled said gate of saidauxiliary power switch.
 10. The LED driver of claim 8, wherein saidfirst unidirectional conduction circuit is coupled in series with acurrent limiter that is configured to limit a charging current of saidcapacitor in order to guarantee that said charging current is less thanan upper limit of a predetermined charging current.
 11. The LED driverof claim 10, wherein said first unidirectional conduction circuit iscoupled in series with said current limiter, and wherein said currentlimiter is configured to turn off said auxiliary power switch when saidcharging current is greater than said upper limit.
 12. The LED driver ofclaim 7, wherein said control circuit is configured to: a) turn off saidmain power switch when said voltage of said main power switch is lessthan a predetermined voltage; and b) turn on said main power switch whena current of said main power switch is saturated.
 13. The LED driver ofclaim 7, further comprising a starting resistor coupled between said DCbus voltage and said supply voltage.
 14. The LED driver of claim 7,wherein said output current sampling resistor is configured to obtain asense voltage signal that represents a current flowing through saidoutput current sampling resistor, and wherein said sense voltage signalis provided to said control circuit for controlling said main powerswitch.